Commits on Dec 14, 2016

1. - #PXC-393: Optimized and accurate way to get mmap file gcache size u… …

   …sing mincore syscall
   
   This patch implements a new information schema table ("WSREP_STATS"),
   which contains the "wsrep_gcache_actual_pool_size" variable that reflects
   the exact amount of memory, which is used by the "gcache" memory pool.
   
   Using a separate information schema table for this variable avoids
   deceleration of regular queries, such as SHOW STATUS.
   
   To determine the actual amount of memory occupied by the gcache,
   the mincore() syscall is used.
   
   At the same time, I limited the size of additional vector, which used
   by mincore() syscall, by dividing of large memory ranges into relatively
   small 512MB chunks, which are passed to mincore() syscall. This introduces
   slight performance loss due to additional syscalls, however, these costs
   is less than 1%, while only the 128 kilobytes of additional memory is
   allocated for mincore()-related vector.
   
   I made sure that the state of gcache pages measured only up to the
   currently achieved "watermark", but not around the entire allocated
   pool - to accelerate scenarios such as
   https://bugs.launchpad.net/percona-xtradb-cluster/+bug/1462674
   
   In addition, measurement of the actual amount of memory occupied
   by the gcache avoids long-term locking of the other gcache operations,
   except when mutex is aquired by the allocation or deallocation of
   overflow pages (in the "Pages" storage). In this case mid-term
   locking is possible – until the code that uses memcore() syscall
   will stop working. However, while we do not create/free the overflow
   pages, a global lock does not happen.

   

   sysprg committed Dec 14, 2016
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